In this type of cordless telephone device, a control circuit is provided so that call is supported, only when a mutual ID number stored in a master telephone set connected to a telephone line coincides with that stored in or more slave telephone sets connected with the master telephone set by radio communication, between the master telephone set and the one or more slave telephone sets.
However, in this type of cordless telephone device based on the conventional technology, when a slave telephone set must be replaced for maintenance, it is necessary to carry out a very complicated work for exchanging a memory (or rewriting a content of memory) in which mutual ID numbers are stored by opening or closing a cabinet for the slave telephone set, and also when a new slave telephone set which can communicate with the master telephone set is extended, it is necessary to carry out a very complicated work for transferring the mutual ID numbers stored in the master telephone set to the new slave telephone set to be stored therein by opening or closing a cabinet for the slave or mater telephone set, and in either case the telephone set must be taken to a dealer or a service man must be called for that purpose, which is very troublesome.